if you have 2 properly tuned 75s, what kind of range should you get?

or is that too subjective of a question.

i need to check my ground--i only have the antenna hooked up to the Or-fab tire carrier with no ground wire.

 

There are many factors involved that could affect range between any two CBs, not only 75 to 75.
- The antenna is the most important. High quality, grounded and part of a properly tuned system. Also, with a Firestik, the top third of the antenna should be above roof line on a hard top.
- While CB freqs are in the upper 26 MHz to Lower 27 MHz range, unlike most HF freqs, the expected transmission/reception range is short - about 5 miles average. That being said, because it is in the HF Band, sometimes you can work a "skip" and talk to fellow CB'ers 100 or more miles away.
- The terrain will play a big part in CB range and causes CB's range to be generally described as Line of Sight (LOS) which means that the two antennas must be able to physically "see" each other to communicate. Buildings, hills, canyons, etc, can all reflect or stop CB signals. On the other hand, if you are at 6,000' and can see your buddy on a valley floor 25 miles away, you will most likely be able to talk to him - because the antennas can "see" each other.
- Bottom line. On the trail, if you can see the JK in front of you and the JK behind you, you will normally be able to talk to them. If the lead JK has just gone around a hill or dropped in to a creek bed, and you can't see them, you most likely won't be able to talk to each other. That's when you start "relaying" messages with JKs who can still see the lead Jeep. All is not lost.

 

Standing wave generation is not a reciprocal function in a co-ax fed antenna system. Easily demonstrated on any well equipped test bench. Do you understand what the term "Return Loss" means? "Reflected Power" is also more correct than "Standing Wave Ratio", since SWR is not what is being measured when when using a "SWR" meter. What is measured is a portion of the voltage component of the reflected signal. It is this voltage that is compared to a portion of the voltage component of the incident signal that is picked off the co-ax by a directional coupler and expressed as a ratio. SWR effects are observed by other means. Exact theory is not too important to the average CB enthusiast since tuning in ignorance is just as effective and will reach the same desired end, unless intelligent thought and action, which does require correct theory, is needed to remedy the problem.

Also you make the assumption, that the radio, nominally a 50 ohm signal source, sees only the impedance of the supposedly tuned (or untuned) antenna. This is not necessarily true. The ground system introduces a complex component in the form of a 2 to 50 ohm impedance to true earth ground. The Jeep lacks sufficient surface area to act as a ground plane at CB frequencies. The Jeep body which is actually a part of the Lower Dipole antenna element is capacitively coupled to earth ground to form the complete antenna system. This capacitive coupling introduces an unpredictable reactive element into the impedance formula. Thus even if the radio sees a 50 ohm impedance, it does not necessarily mean that the antenna is tuned to resonance. In other words, a low SWR does not guarantee the antenna is at (or near) resonance. Conversely, an antenna at resonance may, because of "grounding" deficiencies, present the operator with an apparently unacceptable SWR reading and still perform quite well. Incidentally, a dipole mounted on a Jeep will never present a 50 ohm impedance at resonance. See the link for more detail.

http://www.hottconsultants.com/pdf_files/dipoles-3.PDF

 

Portions of this link will explain in slightly greater detail why and HOW grounding affects the impedance the transmitter may see. Sections concerning common mode currents are particularly interesting.

I advise everyone read this link from its home page through to the end. It debunks most all the Internet Wisdom ever wizzed regarding antenna installation and CB radio in general.

http://www.k0bg.com/common.html#inadequate

 

Yes, I understand. And yes I knew what you were referring to when you said "return loss". Swr is a measurement of forward and reflected rf. (And for those of you that read this, in layman's terms, "reflected" rf means the amount of rf signal that is not being radiated through the antenna and is being bounced back towards the radio via the coax). I was just talking basics here......the swr abbreviation stands for "standing wave ratio". I know there are FAR too many variables to be an absolute when it comes to RF. What looks good on paper, or what is advertised, (usually DB gain) doesn't even come close to being the truth in practical application. Heck even Earth ground conductivity changes from one qth to another, so ALL antenna applications, (even with the same antennas), will perform differently. And when it comes to mobile coms it's a whole different story. But again, "BASICALLY" as a rule, even with all the unknown variables, a person "trys" for the lowest swr, in hopes that in doing so his antenna will be tuned adequately enough to allow the receiver to receive as well as it can.

Oh and btw, if you check on some of the other posts you'll see we've all been referring to K0BG's resource for a number of years now. An invaluable tool to say the least. I personally talked to the man on the air a number of years ago.

 

It occurred to me, at the time I posted that, I may regret adding the link because it looked too familiar. I usually try to back up most of my more contentious rants with some corroboration and, in haste, it seemed like a good one. Getting good, valid information from the Net is difficult, not only from boards like this one (which is much better than other Jeep forums), but from sites we would like to use as reference, such as K0BG's. Even though I used it as a reference, I have doubts as to some material he puts forward. For example, I cannot verify his stand on common mode noise intrusion. That's fuel for another fire, somewhere else, though.

 

I can't be certain, but I believe Don, "goldtr8" got some info about common mode from his site, as I know he put a choke outside his jeep at the antenna feed on his Scorpion screwdriver antenna. And I remember him stating that he had a bit of success by doing so. Now that I think about it, I think there were a couple other HAMs here on the forum that also built a choke at the feed. Weather they all got the info from K0BG's site, I can't say for certain.

 

I made another try, this afternoon, to characterize common mode intrusion and this time had "some" success. I simply used the techniques learned from the "environmental scientists" and constructed a model (test jig) more friendly to the concept. I can, indeed, verify its existence and even crudely quantify it. Simple set up. Spectrum analyzer set at 27 MHz input with a coax cable consisting of three - five foot sections joined by BNC to BNC female barrels. Terminated in a (very untuned) sectional whip antenna. Set a signal generator to 0 dbm to calibrate the SA and then inject by direct contact the RF on to each of the barrel connections. Result seems to indicate the signal by direct injection is 30 to 45 dbm down depending where the injection point is. Closer to the receiver (SA) , the greater the rejection (smaller intrusion) which would seem logical. No measurable signal intrusion with a termination (50 ohm) at the antenna end. The quoted article made me expect more of an effect. From what I see, normal, good wire routing should be sufficient to ward away the evil. It seems to me that common mode radiation into the vehicle is the greater problem. The normal operating environment would supply sufficient "noise" to make common mode noise unimportant if conventional good installation practices are followed.